This invention relates to a hinge with movement limitation for use in orthoses, orthopaedic splints and braces employed at the knee.
Mechanically, the knee is a modified, crossed, four bar linkage comprising the rigid elements femur, tibia and the anterior and the posterior cruciate ligaments. Its axis of rotation moves backwards or posteriorly as the knee is flexed from the fully extended position. The locus or track of the axis of knee rotation is called the "Instant Centre Pathway" which exactly defines the moving path of the centre of knee rotation at any given instant.
In a first, very widely used type, there are two hinge arms each having its own pivot and also each having a set of gear teeth about the periphery of that part which extends between the pivots. The arms are so sized and arranged that the gear teeth mesh between the pivot points, thereby integrating the arm movements. Thus if one arm moves, the other must move as well. This type is generally referred to by those skilled in the art as the geared bi-axial, geared duocentric or geared polycentric type. The latter term is perhaps the most widely recognised. This type of hinge is not at all physiological in the way it moves and its mechanical action is quite unlike that of the human knee. Consequently such a hinge cannot accommodate or track the complex motion of the knee properly.
Another type of hinge design used in orthopaedic splints and braces employed at the knee has two hinge arms, each having its own pivot but in this design there are no gear teeth. Thus, in this type, the arm movements are not integrated and each arm can always move independently without affecting the other. This type of hinge is generally referred to by those skilled in the art as the true bi-axial, true bi-pivotal or simply just bi-pivotal type. It continues to grow in popularity with the realization that such a construction is superior to the others in providing the freedom necessary to accommodate the complex and changing locus of the axis of the knee throughout the entire flexion/extension cycle.
However, a third type, only slightly less popular than the geared polycentric type, has two hinge arms which are joined at and flex about a single pivot. This type is generally referred to by those skilled in the art as the uni-axial, uni-pivotal or monocentric type. Like the geared polycentric hinge, this type is not physiological but because it can be made simply and cheaply, basic designs have frequently found favour in braces and orthoses used in the early phases of treatment and rehabilitation following injury to or surgery on the knee, such as rupture and repair of the anterior cruciate ligament. Most users and manufacturers are aware that uniaxial hinges offer little in the way of physiological tracking or accommodation of natural knee motion. However, the argument made by both groups in favour of using uniaxial hinge usually relates to cost and also propounds that in the acute situations, where most use of this type occurs, ranges of motion are small and activity levels are low.
The origin of single pivot hinges in general is lost in the mists of time but it is found in pre-historic flails used for grain and in early jewellery. The uniaxial hinge has been used in a basic form since the earliest days of the orthotic profession and this probably emerged from its widespread earlier use by makers of military armour.
In 1855, H. H. Smith writing in the American Journal of Medical Science described a true splint but which he called an "artificial limb" which featured a uniaxial hinge at the knee. It does not appear to have had any means for controlling flexion or extension. In 1866, U.S. Pat. No. 58,403 to R. J. P. Goodwin, described a splint primarily for fractures which had a uniaxial hinge and a slot and screw arrangement for compressively locking the arms of the splint in a selected position in what is effectively a continuously variable manner.
In 1889, U.S. Pat. No. 401,933 to W. H. De Camp, described a fracture apparatus for surgeons' splints which had a uniaxial hinge with a body in the form of a disk segment, attached to one hinge arm and provided with a series of openings to receive a pin mounted on an auxiliary spring arm carried on the same pivot rivet as the arms. This arrangement provided flexion stops or discontinuous extension stops but not both at the same time. Free motion of the hinge could be provided by moving the auxiliary spring arm beyond the extent of the disk.
A somewhat similar but simplified arrangement is described in U.S. Pat. Nos. 4,481,941 and 4,531,515 to Rolfes. In these devices a pair of substantially circular hinge plates are at the end of a first hinge arm and a second hinge arm is disposed between them. Both plates have a series of aligned holes but those in the rearmost plate may be threaded. Pins constituting discontinuously variable stop means are passed non-threadedly through the front plate and engage optionally threadedly with the rear plate.
In U.S. Pat. No. 4,738,252, to Friddle et al, a uniaxial joint is described with means for controlling flexion and extension stop positions defined by the length of an arcuate slot acting against a stop pin. In addition, means are provided, in the form of mutually engaging serrated members, for repositioning the arcuate slot. No means for selectively altering the stop position within the arcuate slot, are disclosed.
U.S. Pat. No. 4,982,732 to Morris discloses a uniaxial knee brace having a slotted circular hinge member, 16 discrete radial stops for extension and a further 16 discrete stops for flexion, each stop being provided with its own leaf spring. The stops extend through slots in a cover which bear some resemblance to those of De Camp op cit. Both sets of stops may be individually and selectively moved from a disengaged or parked position in a circular hinge member to an engaged position where they contact abutment stops formed in a circular cam member, thus limiting flexion and extension in a discontinuous manner.
U.S. Pat. No. 5,000,169, to Swicegood et al, employs a serrated indexing ring plate which the authors call a rounded serrated track. Movable, separate stop carriers for discontinuous limitation of flexion and extension are mounted in arcuate slots within the plate. Stops are in the form of pins which engage both a recess in the serrated track and a hinge base plate.